1. Field of the Invention
The present invention relates to bionanotechnology and in particular to a method of fabricating a hybrid microfluidic/nanofluidic device having a gradient structure formed by a modified photolithography technique at the interface between microfluidic and nanofluidic portions of the device and uses thereof.
2. Description of the Related Art
Nanotechnology, electronics and biology are combined in the newly emerging field of bionanotechnology. Nanofabrication of extremely small fluidic structures, such as channels, can be used in bionanotechnology for the direct manipulation and analysis of biomolecules, such as DNA, and proteins at single molecule resolution. For example, the channels can be used for stretching genomic DNA and scanning for medically relevant genetic or epigenetic markers. New insights of understanding the confinement-mediated entropic behavior of biopolymers in ultra-small nanoscale fluidics have just started to emerge.
On the nanometer scale, DNA is a stiff molecule. The stiffness of the molecule is described by a parameter called the persistence length. Despite the relative stiffness of DNA for sufficiently long molecules, it tends to form a disordered tangle of compact random coils in free solution. The conformation of a polymer in free solution has been referred to as a spherical “blob” by the polymer dynamics community. The size of the blob depends on the length of the DNA molecule and the persistence length.
It has been described that in order to uniformly stretch chain-like long DNA, dimensions of nanofluidic structures should be near, in the vicinity of or smaller than the persistence length of double stranded DNA of about 50 nm to about 70 nm. Arrays of up to half millions of nanochannels fabricated over a 100 mm wafer using nanoimprinting lithography (NIL) with sealed channels having a cross section as small as 10 nm by 50 nm to stretch, align and analyze long genomic DNA in a highly parallel fashion, and the resulting have been described in Cao H., Wang J., Tegenfeldt P., Austin R. H., Chen E., Wei W. and Chou S. Y., Fabrication of 10 nm Enclosed Nanofluidic Channels (2002) Applied Physics Letters, Vol. 81, No. 1, pp174. It is challenging to efficiently move long DNA arranged as a blob into the small channels, since it is energetically unfavorable for long biopolymers to spontaneously elongate and enter nanochannels directly from the environment due to the large free energy needed to overcome negative entropy change, as illustrated in FIGS. 1A–1B. For example, a double stranded T4 phage DNA molecule with a length of 169 kilobases will form a Gaussian coil with a radius of gyration (Rg=(Lρ/6)1/2, where L is the length and ρ the persistence length of the DNA), approximately 700 nm in aqueous buffer solution which is many times the width of the opening of the nanochannels. Consequently, problems such as DNA clogging at the junction of nano- and macro-environment have arisen and undermine the performance of conventional nanofluidic devices.
U.S. Patent Application No. 2002/0160365 describes a method for separation of long strands of DNA by length by forcing the molecules to traverse a boundary between a low-force energy region and a high-force energy region. The high-force energy region is a diverse pillar region. The low-force energy region is a larger chamber formed adjacent the high-force energy region.
U.S. Patent Application No. 2002/0072243 describes fabrication techniques using a pattern of sacrificial and permanent layers to define the interior geometry of a fluidic device. A pattern for a fluidic device having microchannels and an array of retarding obstacles is defined in a resist layer. The pattern is produced using lithographic techniques. For electron beam lithography and for deep structures made with photolithography, a hard pattern mask is required to assist in pattern transfer. An inlet chamber, outlet chamber, inlet microchannel, outlet chamber and an array of holes is formed in a sacrificial layer. A ceiling layer is deposited to cover the sacrificial layer. The ceiling layer enters the holes to form closely spaced pillars. The sacrificial layer is removed to form microchannels between the floor and ceiling layers. The pillars act as a sieve or an artificial gel filter for fluid flowing through the system. Steps needed in removing the sacrificial materials, such as heating the substrate up to 200–400° C., limits the use of certain materials. Electron beam lithography has the flexibility to write different patterns, but has low throughput and high manufacturing costs.
It is desirable to provide an improved structure interfacing between microfluidic and nanofluidic components of a device for reducing the local entropic barrier to nanochannel entry and an improved method for fabrication thereof.